This invention is directed to character generation systems and more particularly to character generation systems for creating a character display on a TV receiver.
One of the problems with some prior art character generator systems is that they will not create characters on the screen of a standard residential TV receiver without substantial modifications being made to the receiver. Thus, they use expensive special receivers for character display. Another problem with prior art color character generator systems, is the undesirable color fringing effect which occurs when a standard interlaced 262 1/2 dual field technique is used. This invention is directed to overcoming these and other problems related to prior art character generator systems, particularly those which display characters in color.
More specifically, in setting standards for broadcast color television, special consideration was given to the method of coding the color values and combining it with the monochrome information in a compatible and bandwidth conserving manner. The standards for accomplishing this have been set forth by the NTSC and approved by the FCC.
The general principle utilizes a low visibility subcarrier for transmission of the chrominance information. In effect a system was developed which would introduce a subcarrier, the frequency of which would be an odd harmonic of over half the line scanning frequency. The result is to distribute the energy in the frequency spectrum in areas where substantially no energy normally exists, halfway between the line frequency harmonics.
By using the odd harmonics of 1/2 the line frequency, the subcarrier on line 526 (line 1 of the successive frame) will be 180 degrees out of phase, relative to the previous frame of line 1. This reversal results in low visibility because of the physiological integration of the eyes of the viewer.
One problem that results from the use of this subcarrier relationship is an apparent upward crawl of the odd harmonic, one half the line, frequency pattern. This dot crawl is particularly visible in synthetically generated color pictures such as computer generated graphics and alphanumeric character generators.
A solution to the upward edge crawl can be obtained by changing the number of lines per frame from the standard 525 lines to 526. This results in a very small change in the vertical rate, but not enough to affect most standard color receivers.
This also results in a non-interlaced picture because each successive field now becomes overlayed, line-for-line with the preceding one. Now only half the number of lines appear on the raster as normal. In the case of computer generated graphics, however, often the second field is redundant with the first, and as a result, no loss of resolution occurs. In fact, with computer generated graphics, the appearance is subjectively improved by the use of non-interlaced sync. One of the requirements for these results is that the horizontal frequency be an odd harmonic of the vertical frequency. This results in a 180.degree. phase reversal of the subcarrier, not only from line to line, but now from field to field, resulting in complete invisibility of the subcarrier components in a stationary picture.
Since the number of lines per field must be an odd number, and to maintain compatibility with NTSC standards, 263 lines per field is chosen by this invention as the closest odd number to the standard 262-1/2 lines.
It is an object of this invention to provide a character generator system.
It is also an object of this invention to provide a character generation system for creating the character display on a TV receiver.
It is a further object of this invention to provide a character generation system suitable for creating a non-interlaced character display on a standard residential TV receiver.
It is a still further object of this invention to provide a character generation system for creating a non-interlaced character display in color on a residential TV receiver.